1. Field of the Invention
The present invention relates to an electric device having nanowires, a method of manufacturing such electric device, and an electric device assembly. In particular, the present invention relates to an electric device such as a TFT device that uses nanowires as conductive channels, and a method of manufacturing such electric device.
2. Description of the Related Art
A thin film transistor (TFT) is a switching device used in various electronic devices. Known transistor devices that constitute the TFT include silicon-based transistors (single crystal, polycrystal, and amorphous), compound semiconductor transistors (III-V group, II-VI group, and IV-IV group), and organic transistors (low molecular, and polymer).
Of those, silicon-based transistors, which use silicon for a semiconductor layer, have the following characteristics:
1) Their material, silicon, is infinitely abundant on the surface of the earth.
2) Doping can be used to build p-type and n-type structures.
3) An oxide of silicon SiO2 can be used for forming a high quality dielectric film.
4) The carrier mobility is high (single crystal: up to 103 cm2/Vs, polycrystal: up to 102 cm2/Vs, amorphous: up to 1 cm2/Vs), which leads to excellent transistor performance.
However, silicon-based transistors need a large-scale manufacturing facility and a huge apparatus for film formation in the device fabrication process through chemical vapor deposition, or other similar film forming methods, at high temperature under vacuum. Accordingly, cost reduction and process simplification are problems to be solved.
Compound semiconductor transistors are transistors that use for a semiconductor layer a compound (GaAs, SiC or the like) formed of a plurality of elements. Compound semiconductor transistors have much higher carrier mobility than that of silicon-based transistors, and exhibit various characteristics including low-power drive in high-frequency bands, photoreactivity, and microwave emission, depending on what compound is employed. Despite those diversity of characteristics, compound semiconductor transistors have limited uses since their materials are expensive and their device fabrication process is a large-scale, complicated process comparable to that of silicon-based transistors described above.
Organic transistors are transistors that use for a semiconductor layer an organic substance (pentacene and the like as a low-molecular weight organic substance, PEDOT (polyethylene dioxythiophene) and the like as a polymer organic substance). Organic transistors, especially polymer organic transistors, be produced by a coating process in forming a film, and are adaptable to simple, low-cost fabrication of devices in mass quantities through ink jet or roll-to-roll processes. However, organic transistors are significantly low in carrier mobility (up to 0.1 cm2/Vs), which determines the performance of a transistor, compared to that of silicon-based transistors, and are said to require significant progress with regard to material and manufacture process for being applicable to various electronic devices.
As described, none of the prior art TFT devices has succeeded in having high transistor performance and in being obtainable by a simple, low-cost device fabrication process, and development of such a transistor has been desired. Nanowire TFTs are attracting attention as a next-generation TFT device that meets this demand.
Nanowire TFTs are TFTs that use nanowires having a high aspect ratio of a diameter of several nm to several hundreds nm and a length of several μm to one hundred μm, for a semiconductor layer, which connects a source electrode and a drain electrode in a transistor device. Nanowire TFTs that use nanowires of high crystallinity are expected to have high transistor performance. When the nanowire diameter is 5 nm or shorter, in particular, a quantum effect is brought about, so that higher performance such as a single electron transistor and high mobility can be expected. In addition, nanowire TFTs can be manufactured by a simple, low-cost device fabrication process since it merely requires orientation/arrangement control of nanowires on a substrate to build a TFT structure. Nanowire TFTs are therefore expected to embody a next-generation TFT having both a high transistor performance and being obtainable by a simple, low-cost device fabrication process, which has been unattainable with the conventional TFT technology.
Examples of channel materials to be used for field effect transistors using nanowires as a channel, which are at present under development, include: carbon nanotubes, Si nanowires, Ge nanowires; semiconductor materials including Group III-V compound semiconductors such as GaAs and InP, and Group II-VI compound semiconductors such as CdS.
A known nanowire synthesis method is, for example, the Vapor-Liquid-Solid (VLS) growth. Japanese Patent Application Laid-Open No. H10-106960, for example, discloses a silicon nanowire synthesis method. The first step of this method is to deposit gold onto a silicon wafer surface followed by heating in a silane gas atmosphere, thereby forming a molten alloy of silicon and gold on the silicon wafer surface. With the alloy working as a catalyst, the silane gas is decomposed and silicon nanowires grow.
The thus obtained silicon nanowires have significantly high crystallinity in the axial direction. The article on pages 274-278 of “Nature” vol. 425 issued Sep. 18, 2003 discloses that such nanowires, when used for a semiconductor layer in a TFT, give the TFT as high transistor performance as that of from polycrystalline silicon TFTs to single crystal silicon TFTs.
Also, as for a device fabrication method, nanowire TFTs can be manufactured by a coating method, in which nanowires are dispersed into a solution to be applied to a substrate. However, simply applying a nanowire solution to a substrate is not enough and has to be accompanied by orientation/arrangement control of each nanowire. The article on pages 274-278 of “Nature” vol. 425 issued Sep. 18, 2003 or U.S. Pat. No. 6,872,645 reports a technique of orienting/arranging nanowires by causing a nanowire solution to flow over a substrate. Forming nanowires by coating like this has process advantages such as being simple, low-cost, and fit for mass production.
In addition to the excellent TFT performance as described above, nanowire TFTs are characterized by being highly resistant against bending and by possibility of being manufactured by a normal pressure, normal temperature fabrication through a thoroughly liquid process. Accordingly, when formed on a plastic substrate such nanowire TFTs make it possible to form switching devices that have higher TFT performance than would ever be possible with conventional TFT technology as well as a sophisticated electrical processing circuit, on a flexible substrate through a low-cost process. For instance, in the case of TFT substrates for flat panel displays such as liquid crystal displays and organic EL displays, the silicon nanowire TFT technology makes it possible to fabricate a plurality of TFTs for each pixel and, furthermore, allows nanowire TFTs to constitute a driver circuit of a display peripheral circuit or the like. Flexible displays that have high image quality and can be driven at low power can thus be manufactured at low temperature.
Trimming is known as a method of modifying characteristics of electric devices. Trimming is commonly used in such electric devices as resistors, capacitors, and coils. A resistor is trimmed to reduce the width of the resistor or to change the effective length of the resistor, thereby raising the resistance value to a given level. A capacitor is trimmed to remove some of electrodes, thereby being capable of adjusting the capacitor by reducing the capacity.
Methods of trimming include, in addition to one that uses a laser beam, conventionally known methods such as sandblasting, one that uses a diamond rotary blade, electronic beam machining, ultrasonic machining, and anodization. Laser trimming, in particular, has the following advantages:
1) The lack of mechanical contact with an object to be worked on results in less contamination and damage to the object.
2) An object housed in a transparent package can be trimmed from outside of the package.
3) An object can be machined to have a minute width.
4) Resistors of high-density integrated circuits can be trimmed.
5) The precision and reproducibility are high.
6) A trimmed object causes little change with time.
Laser trimming is therefore suitable for trimming of electric devices, and the mainstream of electric device trimming is laser trimming using a YAG laser, a CO2 laser, or the like. Laser trimming cannot be performed on TFT devices since it only makes TFT devices physically unusable.
There are two types of trimming: element trimming and functional trimming. Functional trimming, in which a printed resistor is trimmed to have the optimum resistance value while the overall circuit function is observed, is widely employed at present.
The nanowire TFTs described above are arranged by a simple process of causing a solution in which nanowires are dispersed to flow over a substrate and thus depositing nanowire TFTs. On the other hand, the simple process allows nanowires to be arranged irregularly, which leads to characteristics fluctuations. Causes of the irregular arrangement of nanowires include a fluctuation in number of effectively working nanowires, a fluctuation in length of the nanowires between source and drain electrodes due to deviation of the angle between the nanowires and the electrodes from being perpendicular, and an overlap of nanowires. These fluctuations related to arrangement of nanowires can make nanowire characteristics depart from specification values.